Methods and systems for controlling the products of combustion

ABSTRACT

The present invention relates to methods and systems for controlling a combustion reaction and the products thereof. One embodiment includes a combustion control system having an oxygenation stream substantially comprising oxygen and CO 2  and having an oxygen to CO 2  ratio, then mixing the oxygenation stream with a combustion fuel stream and combusting in a combustor to generate a combustion products stream having a temperature and a composition detected by a temperature sensor and an oxygen analyzer, respectively, the data from which are used to control the flow and composition of the oxygenation and combustion fuel streams. The system may also include a gas turbine with an expander and having a load and a load controller in a feedback arrangement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No. 13/062,442 filed Mar. 4, 2011 entitled METHODS AND SYSTEMS FOR CONTROLLING THE PRODUCTS OF COMBUSTION, which is the National Phase application of PCT/US2009/055544 filed Aug. 31, 2009 entitled METHODS AND SYSTEMS FOR CONTROLLING THE PRODUCTS OF COMBUSTION, claims the benefit of U.S. Provisional Application No. 61/105,331 filed Oct. 14, 2008.

FIELD OF THE INVENTION

Embodiments of the invention relate to methods and systems for controlling the products of combustion. More particularly, methods and systems for obtaining substantially stoichiometric combustion in an oxy-fuel type combustion reaction are provided.

BACKGROUND OF THE INVENTION

This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

Some approaches to lower CO₂ emissions include fuel de-carbonization or post-combustion capture. However, both of these solutions are expensive and reduce power generation efficiency, resulting in lower power production, increased fuel demand and increased cost of electricity to meet domestic power demand. Another approach is an oxy-fuel gas turbine in a combined cycle. However, there are no commercially available gas turbines that can operate in such a cycle.

The original oxy-fuel concept is based on the combustion of hydrocarbons with pure oxygen, which results in extremely high temperatures. Such high temperatures lead to problems in combustor life and also the development of Polycyclic Aromatic Hydrocarbons (PAHs), which lead to soot production. Numerous solutions to these issues have been attempted with varying levels of success, including using carbon dioxide as a mass flow gas through the turbine instead of air. However, this approach and others require specialized turbine equipment that is not yet commercially available.

U.S. Pat. No. 5,724,805 discloses a power plant having a gas turbine fueled by hydrocarbons mixed with an oxygen/carbon dioxide mixture. However, the disclosure states that the O₂/CO₂ mixture has more O₂ than air and therefore burns at very high temperatures requiring a large combustor chamber to allow time for the combustion gasses to gradually cool so less carbon monoxide (CO) is produced. As such, a specialized, non-standard combustor must be used for the techniques in the '805 reference to be practiced.

As such, there is still a substantial need for methods and systems to effectively control the temperature and composition of combustion product streams.

SUMMARY OF THE INVENTION

One embodiment of the present invention discloses a combustion control system. The combustion control system comprises a combustor consisting of at least a primary combustion zone and a burnout zone; a high concentration carbon dioxide (CO₂) supply (the diluent supply); an oxygen supply stream configured to combine with at least a first portion (the primary diluent flow) of the high concentration CO₂ stream to form an oxygenation stream substantially comprising oxygen and CO₂ and having an oxygen to CO₂ ratio; and a combustion fuel stream with a flow and a composition. The system further includes a combustor configured to mix and combust the oxygenation and combustion fuel streams within a primary combustion zone and a burnout zone in which a second part of the diluent supply (the secondary diluent) is added to form combustion products stream with a temperature and a composition; at least one temperature sensor configured to measure the temperature of the combustion products stream after the exit of the combustor, wherein the temperature of the combustion products stream is used to adjust the flow rate of the secondary diluent to obtain the desired temperature at the exist of the combustor; and at least one oxygen analyzer configured to measure the amount of oxygen in the composition of the combustion products stream, wherein the amount of oxygen in the combustion product is used to regulate the flow rate of the oxygen supply stream to achieve substantially stoichiometric combustion.

In some embodiments, the combustion fuel stream may be comprised of at least a high quality fuel gas stream, a low heating value fuel gas stream, and optionally, a high concentration CO₂ makeup stream. The separate streams may be operatively connected to a summation controller connected to the flow controllers for the individual streams to control the flow and composition of the combustion fuel stream to regulate the temperature of combustion and avoid flame burnout. In some embodiments, each of the streams may be operatively connected to a flow controller controlled by a central control system.

In additional embodiments, the combustor may include a first mix zone configured to mix the first portion of the high concentration CO₂ stream and the oxygen supply stream to form the oxygenation stream; a primary combustion zone configured to house the combustion reaction which produces the combustion products stream; and a burnout zone configured to deliver the second portion of the high concentration CO₂ stream to the combustor to regulate the temperature of the combustor and the combustion products stream. In one exemplary embodiment, a catalyst is added to the initial high temperature combustion zone to catalyze the combustion reaction. In another alternative embodiment, the second mix zone may be configured to pre-mix the oxygenation and combustion fuel streams or concurrently mix the streams with the combustion reaction.

Another embodiment of the present invention discloses a combustion control method. The method comprising providing a high concentration CO₂ stream, an oxygen supply stream, and a combustion fuel stream, wherein each stream has a flow rate and a composition; combining at least a first portion of the high concentration CO₂ stream and oxygen supply stream to form an oxygenation stream; combusting the oxygenation stream and the combustion fuel stream in a combustor to form a combustion products stream with a temperature and a composition; sensing the oxygen content of the combustion products stream; and adjusting the flow rate of the oxygen supply stream until the combustion products stream is substantially stoichiometric.

In a third embodiment of the present invention, a combustion system is provided. The combustion system includes a combustion fuel stream comprising substantially hydrocarbons and carbon dioxide (CO₂) and having an initial fuel to CO₂ ratio; an oxygenation stream comprising substantially oxygen and carbon dioxide (CO₂), wherein the combustion fuel stream and the oxygenation stream are combined to form the combustion reactants stream having a combined fuel to oxygen ratio regulated to meet a desired equivalence ratio (defined as the ratio of the actual fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizer ratio) and a combined initial CO₂ to fuel ratio regulated to provide a desired combustion temperature within the primary combustion zone; a secondary diluent comprising substantially carbon dioxide (CO₂); and a combustor configured to combust the combustor inlet stream to produce the primary combustion product comprising substantially water and carbon dioxide, wherein the primary combustion product is mixed with the secondary diluent to form a combustion products stream having a temperature and a final CO₂ to fuel ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the present invention may become apparent upon reviewing the following detailed description and drawings of non-limiting examples of embodiments in which:

FIGS. 1A-1E illustrate four alternative exemplary control schematics in accordance with certain aspects of the present invention;

FIG. 2 illustrates a schematic of an exemplary combustor as it might be configured for use in the alternative exemplary systems of FIGS. 1A-1E.

FIG. 3 is an exemplary flow chart of a method of operating the system of FIGS. 1A-1E;

DETAILED DESCRIPTION

In the following detailed description section, the specific embodiments of the present invention are described in connection with preferred embodiments. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present invention, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the invention is not limited to the specific embodiments described below, but rather, it includes all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

The term “stoichiometric combustion,” as used herein, refers to a combustion reaction having a volume of hydrocarbons (e.g. fuel) and a volume of oxygen, where the volume of oxygen is just enough to combust or burn all or nearly all of the volume of hydrocarbons to produce a volume of combustion products having almost no oxygen remaining and almost no hydrocarbons remaining.

The term “primary residence time,” as used herein, is the time required in a combustor to produce a combustion products stream at about equilibrium conditions at the local conditions of pressure and temperature.

Embodiments of the present disclosure provide combustion processes and systems designed for oxy-fuel combustion in a gas turbine. Preferred embodiments of the invention address problems associated with high temperature oxy-fuel combustion, such as the development of Polycyclic Aromatic Hydrocarbons (PAH's), which lead to soot production and production of problematic combustion products such as oxygen and carbon monoxide (or similar products of incomplete combustion). One exemplary embodiment of the combustion system design includes a high concentration carbon dioxide (CO₂) stream that is divided into at least a primary diluent and secondary diluent and an oxygen supply stream configured to combine with the primary diluent to produce an oxygenation stream substantially comprising oxygen (O₂) and carbon dioxide (CO₂) (e.g. “synthetic air”). The system further includes a combustion fuel stream and a combustor, wherein the combustor, which consists of at least a primary combustion zone and a burnout zone, is configured to mix and combust the combustion fuel stream and the oxygenation streams in a substantially stoichiometric combustion reaction to form a primary combustion products stream substantially comprising water (steam) and CO₂. In addition, the primary combustion product stream may be diluted with the secondary diluent to form a secondary combustion product stream.

Note, that in some embodiments, a high pressure combustion (e.g. greater than about 10 atmospheres) process may be utilized. The adiabatic flame temperature in the primary combustion zone can be controlled by varying the ratio of CO₂ mixed with the oxygen when forming the oxygenation stream. The temperature of the combustion products stream may be controlled independently to obtain the desired temperature or other properties of the combustion products at the exit of the combustor. As such, in some embodiments, the system will include a temperature sensor for measuring the combustion products stream and the percentage amount of CO₂ in the combustion products stream may be increased to decrease the temperature of the combustion products stream or decreased to increase the temperature.

In some embodiments of the present invention, CO₂ and oxygen are mixed to make a “synthetic air” (e.g. an oxygenation stream). The amount of CO₂ mixed with the oxygen provides a way to control the temperature of the primary combustion product stream and also another variable to help control the composition of the products of the combustion. The combustor design may include quench ports to provide additional CO₂ to the burnout zone to prevent the high temperatures of combustion from impinging on the combustor can. Additional embodiments of the system include a control system that measures the amount of hydrocarbon going to the combustor and calculates and controls the correct amount of oxygen needed for the combustion. The control system will also utilize feedback from instrumentation on the products of combustion to update the oxygen supply stream flow controller to ensure the desired combustion is achieved to provide the correct amount of oxygen to the oxygenation stream. A catalytic post combustion step is also optionally provided, which may be required depending on the hydrocarbon mixture that is used for the combustor. This catalytic step will reduce the oxygen in the primary combustion products stream down to the low levels required to avoid serious corrosion problems in the enhanced oil recovery (EOR) facilities.

Some embodiments of the present invention include methods of operating an oxy-fuel combustion system to provide the benefits and advantages of the disclosed systems. For example, one embodiment includes combining the combustion fuel stream with the oxygenation stream and combusting these streams in a combustor to form the combustion products stream. The combustion fuel and oxygenation streams may be pre-mixed or concurrently combined and combusted and may include a catalyst in some embodiments, depending on the composition and rates of the various streams. The method further includes sensing or detecting the temperature and/or composition of the combustion product stream and adjusting the flow rate of at least one of the combustion fuel and oxygenation streams until the combustion is at substantially stoichiometric conditions.

The methods and systems of the present disclosure may be utilized in a variety of applications, including a combustion gas turbine burner system. The gas turbine may be an integrated turbine operating on a single shaft, a multiple-shaft system, or a non-integrated system with an external burner, and may even be utilized with an independent compressor and hot gas expander, depending on the temperatures, volumes, and other variables of the particular system. The methods and systems may be utilized to beneficially increase combustion efficiency (e.g. reduce the amount of unburned or partially combusted fuel and/or oxygen) and provide greater control of turbine inlet temperature across a range of load conditions.

At least one benefit of the disclosed systems and methods includes the flexibility to use a commercial gas turbine combustion can type system with an oxy-fuel/co-generation type of system, such as the ultra-low emission power generation systems and processes disclosed in US2011/0000221. By controlling the amount of CO₂ mixed with the oxygen to form the oxygenation stream, the temperature and composition of the primary combustion products stream can also be controlled. Application of the disclosed systems and methods may avoid the need to develop a new combustor can for a gas turbine, permitting the use of “off-the-shelf” gas turbine combustor technology in such a system.

The combustor utilized could be similar to those used in the gasification process where oxygen and hydrocarbons react in a reducing atmosphere using steam to moderate the temperature. In the present invention, CO₂ would be used in place of the steam to moderate the temperature. Using steam is expensive and would also result in the formation of additional hydrogen in the products of combustion which is not desired in the present cycle. By mixing the CO₂ with the oxygen, it may also be possible to use a more conventional diffusion type combustor similar to those used in existing gas turbines where CO₂ would be used instead of air to cool the combustion liners. Combustion at near stoichiometric conditions is preferred to eliminate the cost of excess oxygen removal.

Embodiments of the present invention provide additional benefits. The present systems and methods enable an operator or automated system to control the temperature of the primary combustion zone separately from the temperature of the combustion product stream and control the products of combustion, such as limiting the amount of corrosive carbon monoxide and oxygen in the combustion product, therefore enabling the use of the combustion product in enhanced oil recovery (EOR) operations, which require the elimination of such corrosive components. Additionally, the disclosed systems and methods can adapt to changes in the quality of available fuel gas. For example, if a low heating value (e.g. less than 40 percent (%) methane) fuel gas is provided, such as from a low quality gas reservoir or a reservoir after CO₂ breakthrough, the systems and methods can adjust the ratio of oxygen in the oxygenation stream and/or add or increase the amount of high quality fuel gas or spiking fuel gas (e.g. hydrogen gas) to the combustion process to maintain the proper temperature and composition in the combustion products stream.

Referring now to the figures, FIGS. 1A-1D illustrate four alternative exemplary control schematics in accordance with certain aspects of the present invention. In particular, FIG. 1A is a basic exemplary system. The system 100 includes a high concentration carbon dioxide (CO₂) stream 102 that may be split into at least a primary diluent stream 102 a and a second diluent stream 102 b, and an oxygen supply stream 104, which may be combined with the primary diluent stream 102 a to form an oxygenation stream 106 having an oxygen to CO₂ ratio. A combustion fuel stream 108 is also provided, which may be comprised substantially of methane (CH₄) or may include a mixture of light hydrocarbons, heavier hydrocarbons, hydrogen (H₂), and inert gasses, such as carbon dioxide, depending on the source. A combustor (e.g. combustor can) 110 is also provided, which in the preferred embodiment is divided into two parts, a primary combustion zone 110 a and a burnout zone 110 b, and which is configured to receive at least the oxygenation stream 106 and the combustion fuel stream 108, mix and combust the oxygenation and combustion fuel streams 106 and 108 in the primary combustion zone 110 a at a desired flame temperature and primary residence time inside the combustor sufficient to produce a hot products stream (not shown) near equilibrium conditions and then dilute the hot products stream with the secondary diluent within the burnout zone 110 b to form the combustion products stream 112 a, which may be fed into an expansion device 111 (e.g. a gas turbine or hot gas expander), which is operatively connected to a load controller 111′ to form an expanded products stream 112 b. The expanded products stream 112 b may be split to form stream 113, which may form at least a portion of the high concentration CO₂ stream 102 and a secondary stream 128, which may be utilized for enhanced oil recovery (EOR), sequestration, or another purpose. The system 100 further includes at least one of a temperature sensor 114 and an oxygen analyzer 126 configured to measure the temperature and oxygen content, respectively, of the combustion products stream 112 a or the expanded products stream 112 b. Temperature data from the temperature sensor 114 is used to control the flow rate of the secondary diluent stream 102 b and to regulate the temperature of the combustion products stream 112 a. The flow rate of the oxygen supply 104 is adjusted in proportion to the flow rate of the combustion fuel supply 108. Oxygen data from the oxygen analyzer 126 is used to adjust the proportioning factor of the flow rate of the oxygen supply stream 104 to the combustion fuel supply 108 until a substantially stoichiometric combustion is achieved.

Still referring to FIG. 1, the system 100 further includes a central controller 115 operatively connected to a first flow controller 116 a for controlling the primary diluent 102 a; a second flow controller 118 for controlling the oxygen supply 104; a third flow controller 120 for controlling the combustion fuel stream 108; and a fourth flow controller 116 b for controlling the secondary diluent stream 102 b. The central controller 115 may also be connected to the temperature sensor 114 and the oxygen sensor 126 to determine the amount of unburned oxygen in the combustion products stream 112 a or the expanded products stream 112 b and use those measurements to control the flow of the oxygen supply stream 104. The central controller 115 may also control the flow rate of the combustion fuel stream 108 and the oxygen supply stream 104 to maintain desired stoichiometry as the load condition changes.

The high concentration carbon dioxide (CO₂) stream (or “diluent supply stream”) 102 may come from any convenient source. For example, at least a portion of the diluent supply stream 102 may be derived from diverting or splitting at least a portion of the expanded products stream 112 b via recycle stream 113. However, the system 100 may be located near another source of high concentration CO₂, such as an external pipeline network, a high CO₂ gas well, a gas treatment plant, or other source. In addition, recycle stream 113 may include some treatment, such as a filtering system like a membrane, mole sieve, absorption, adsorption, or other system to remove potentially dangerous or undesirable components, such as un-reacted oxygen or hydrocarbons. In particular, if the oxygen analyzer determines that the expanded products stream 112 b has high levels of oxygen, then the expanded products stream 112 b should not be used as a diluent, like in the secondary diluent stream 102 b. Similarly, high levels of unreacted hydrocarbons may also be unacceptable, depending on the combustor 110 and may need to be removed or separated before use as a secondary diluent stream 102 b. However, it is preferred and intended that the combustion product stream 112 a has undergone a substantially stoichiometric combustion, so it should have less than about 3.0 volume percent (vol %) oxygen, or less than about 1.0 vol % oxygen, or less than about 0.1 vol % oxygen, or even less than about 0.001 vol % oxygen and less than about 3.0 volume percent (vol %) hydrocarbons, or less than about 1.0 vol % hydrocarbons, or less than about 0.1 vol % hydrocarbons, or even less than about 0.001 vol % hydrocarbons.

The secondary stream (or “remainder stream”) 128 may be used for sales, used in another process requiring high concentration carbon dioxide, or compressed and injected into a terrestrial reservoir for enhanced oil recovery (EOR), sequestration, or another purpose. Like with recycle stream 113, stream 128 may need to undergo some conditioning before use to remove potential contaminants or reactants like nitrogen oxides (NOx) or oxygen. Again, it is preferred and intended that stream 104 includes substantially no nitrogen, and that stream 112 a has undergone a substantially stoichiometric combustion, so it should have less than about 3.0 volume percent (vol %) oxygen, or less than about 1.0 vol % oxygen, or less than about 0.1 vol % oxygen, or even less than about 0.001 vol % oxygen and less than about 3.0 volume percent (vol %) NOx, or less than about 1.0 vol % NOx, or less than about 0.1 vol % NOx, or even less than about 0.001 vol % NOx.

The oxygen supply stream 104 may be provided by an air separation unit (ASU) or other process or system providing high purity oxygen. The separated nitrogen may be used in another related process, such as in a nitrogen injection well as disclosed in US2011/0000221. In one exemplary embodiment, the oxygen supply stream 104 may include from about 90 vol % to about 99.9 vol % oxygen with the remainder argon and may include trace amounts of nitrogen and carbon dioxide. In another exemplary embodiment, the oxygen supply stream may include from about 95 vol % to about 96 vol % oxygen with about 4 vol % to about 5 vol % argon and less than about 0.2 vol % carbon dioxide.

The central controller 115 may be any type of control system configured to receive data inputs, such as flow rates and compositions, and send signals to control flow rates via, for example, valves, pumps, compressors, and any other device that may be used to adjust a flow rate. In one exemplary embodiment, the central controller 115 may include a programmable computer having user input devices such as a keyboard and mouse, user output devices such as a monitor and speakers, and may operate using active memory (RAM), and be operably connected to hard disk drives, optical drives, network drives, and databases via a LAN, WAN, Wi-Fi, or other external network.

The flow controllers 116 a, 116 b, 118, and 120 may include programmable automated controllers for receiving and processing signals from the central controller 115, and may be operably connected to flow valves or vanes, vents, or other means of increasing or decreasing the flow of a substantially gaseous stream. Additionally, in one exemplary embodiment, the flow controllers 116 a, 116 b, 118, and 120 may be operably connected to flow and/or composition sensors, which may provide additional data input, such as to verify changes in the flow rates of the respective streams controlled by the flow controllers. In order to maintain flame stability and effective control, it may be beneficial to utilize a high speed controller for any or all of the controllers 116 a, 116 b, 118, and 120.

Although flow controller 116 b may be an active sensor as described above, the flow rate of the secondary diluent stream 102 b may be uncontrolled in one exemplary embodiment. For example, the combustor 110 may include a liner having one or more quench ports (e.g. dilution holes) with a particular pattern and hold size designed to provide dilution and control temperatures in the combustor 110. Hence, the flow rate of the secondary diluent stream 102 b may be primarily dependent upon the hardware design of the quench ports in the combustor 110 and the pressure, temperature and composition of diluent supply stream 102. Additionally, the flow controller 116 b may still be useful for shutting off the flow of secondary diluent 102 b in case of shut down, contamination of the secondary diluent 102 b, or some other reason. In some embodiments, the central controller 115 may further include two out of three voting for certain sensors, such as the temperature sensor 114 and the oxygen analyzer 126. The control system, including the central controller 115 may also be configured with at least one safety interlock and/or shutdown logic and an alarm if the system 100 gets out of control to protect the downstream machinery.

The temperature sensor 114 may be a single sensor or may additionally include a backup sensor for redundancy or an array of sensors in and around the combustion products stream 112 a or the expanded products stream 112 b to ensure accurate temperature readings. Any type of appropriate temperature sensor may be used, although the sensor chosen should have a high resistance to heat and be able to effectively operate at temperatures at or above about 2,000 degrees Fahrenheit (° F.) or even above about 2,200° F. In some exemplary embodiments of the described inventive system 100, the temperature sensor 114 may send data directly to the CO₂ flow controller 116 b, or may send data to the central controller 115, which then controls the response of the flow controller 120. Alternatively, the temperature sensor 114 may also send data directly to the combustion fuel stream flow controller 120. Additionally and alternatively, the temperature sensor 114 may take data from inside the combustor 110 near the exhaust or downstream of the burnout zone 110 b after exiting, at multiple locations along the combustion products stream 112 a and expanded products stream 112 b, or some combination thereof. The temperature of the streams 112 a and 112 b should be limited to within certain operating parameters, which will depend highly on the equipment in use, the type of combustion fuel stream and other input streams available, the potential uses for the remainder stream 128, and other factors.

Generally, the temperature in the primary combustion zone 110 a should be below about 3,500° F. to avoid NOx production and because most commercial combustors 110 cannot operate above such temperatures, but this limitation may be set higher if the material of the combustor 110 can operate at higher temperatures and there is no nitrogen in the system 100. The temperature is preferably less than about 2,500° F. at the inlet of the expander 111. Such high temperatures also contribute to the formation of undesirable Polycyclic Aromatic Hydrocarbons (PAH's), which lead to soot production. However, the temperature in the primary combustion zone 110 a must be sufficiently high to avoid flame burnout, which is done by regulating the oxygen to CO₂ ratio based on the temperature of the reactants entering the primary combustion zone and the heat release available from the specific fuel 108 and sufficiently high to effectively combust substantially all of the oxygen (O₂) and hydrocarbons (e.g. stoichiometric combustion temperature) to produce the expanded products stream 112 b requiring only limited conditioning before use in enhanced oil recovery (EOR) or as a diluent in the system 100. For many cases, the preferred temperature of the combustion product stream 112 a will be from at least about 1,500° F. to at most about 2,500° F. or from at least about 1,600° F. to at most about 1,900° F. For many cases, the preferred adiabatic flame temperature within the primary combustion zone will be from at least 2,450° F. to at most 3,500° F. unless improved materials of construction and no nitrogen is present in the combustion reactants in which case the upper limit may be increased.

The oxygen analyzer 126 may be a single sensor, may include additional sensors for redundancy, or an array of sensors at multiple locations to ensure accurate measurements. For example, a plurality of lambda or wideband zirconia O₂ sensor may be used to provide feedback to one of the central controller 115 and/or the oxygen supply stream flow controller 118. If the lambda sensor is used, the central controller 115 may be configured to dither the ratio of the fuel in the combustion fuel stream 108 to the oxygen in the oxygen supply stream 104 as the oxygen content of the combustion products stream 112 a varies from a stoichiometric coefficient below 1.0 to above 1.0. The dithering process is similar to those used in the automotive industry for internal combustion engines. In any case, the oxygen content of the combustion products stream is preferably low, from less than about 3.0 volume percent (vol %) to less than about 1.0 vol % to less than about 0.1 vol % to less than about 0.001 vol %. If the amount of oxygen is too high, then the flow rate of the oxygen supply stream 104 is reduced. In turn, this may lower the flame temperature, as discussed above, requiring an adjustment of the flow of the combustion fuel stream 108.

FIG. 1B illustrates the basic exemplary system as shown in FIG. 1A, but with additional, optional features configured to further treat or condition the products streams 112 a and 112 b. As such, FIG. 1B may be best understood with reference to FIG. 1A. The system 140 includes all of the features disclosed with respect to FIG. 1A, but further includes a post-combustion catalysis apparatus 146 configured to reduce the oxygen and carbon monoxide content in the products streams 112 a and 112 b and a combustion fuel bypass stream 142 with a flow and a composition and having a flow controller 144 for controlling the flow rate of the combustion fuel bypass stream 142. The oxygen analyzer 126 may be operatively connected to the flow controller 144 directly or indirectly via the central controller 115. Additional flow controllers and oxygen analyzers (not shown) may be required in certain specific embodiments where the combustion fuel bypass stream 142 is split or stream 128 is looped, as discussed further below.

The catalysis apparatus 146 may be a single device or a plurality of devices in parallel or series, but is preferably a small device requiring only a small amount of power to operate. In particular, the catalysis apparatus 146 may be a carbon monoxide reduction catalyst and/or an oxygen reduction catalyst that is normally used in Heat Recovery Steam Generators (HRSG's) to meet emissions requirements. Such a system is generally not designed to remove large amounts of oxygen, but if significant amounts of oxygen remain in the expanded products stream 112 b, the expanded product stream 112 b may need to recycle through the catalysis apparatus 146 more than once before it is compressed and injected for enhanced oil recovery (EOR). As such, in some embodiments, another oxygen analyzer (not shown) may be needed after the catalysis apparatus 146 to ensure that the injection stream 128 is sufficiently low in oxygen (e.g. less than about 0.5 volume percent (vol %) oxygen or less than about 0.1 vol %) to avoid corrosion of the compression and injection equipment and avoid souring the reservoir by injecting oxygen that can react with the hydrocarbons remaining in the reservoir.

The combustion fuel bypass stream (e.g. second portion of the combustion fuel stream) 142 is configured to be combined with the expanded products stream 112 b downstream from where recycle flow stream 113 is divided from the expanded product stream 112 b, and is preferably introduced upstream from the catalysis apparatus 146 so that the additional hydrocarbons may be used in the catalysis apparatus 146 to improve oxygen removal efficiency. However, in some alternative embodiments, the bypass stream 142 may be split and introduced before and after the catalysis apparatus 146. In the embodiment where the EOR stream 128 is looped back to the catalysis apparatus 146, it may be beneficial to introduce a portion of the bypass stream 142 into the EOR stream 128 before looping it back to the catalysis apparatus 146. Beneficially, the bypass stream 142 is configured to reduce the volume percent of oxygen in the EOR stream 128 before compression and injection into a reservoir to substantially avoid corrosion of injection and compression equipment and souring the hydrocarbons remaining in the injection reservoir.

FIG. 1C is an illustration of a third exemplary embodiment of the system of FIG. 1A, which may or may not include the features disclosed in the illustration of the embodiment of FIG. 1B. As such, FIG. 1C may be best understood with reference to FIGS. 1A and 1B. The system 150 includes a hydrocarbon analyzer 152 configured to measure the amount of hydrocarbons in the composition of the products streams 112 a and/or 112 b, a high quality fuel gas supply 108 a controlled by a flow controller 154, and a low heating value fuel gas supply 108 b controlled by a flow controller 156. Flow controller 156 may be directly connected to the hydrocarbon analyzer 152 and/or may be connected via central controller 115. The flow controllers 154, 156, and optionally 120 may be operatively connected to a summation controller 158, which may be connected to the central controller 115 directly or via oxygen supply controller 118.

The high quality fuel gas stream 108 a may be comprised of substantially pure methane (e.g. about 99 vol %) and alternatively may comprise a “spiking” fuel gas such as hydrogen, higher hydrocarbons (C₃+) or any combination thereof. The composition of the high quality fuel gas stream 108 a will primarily vary depending on the needs of the system 150 and the availability of various fuel types, but preferably will not include significant quantities of inert gases (e.g. nitrogen, carbon dioxide, etc.) or acid gases (e.g. sulfur dioxide, hydrogen sulfide, etc.). High quality fuel gas stream 108 a may be from any reasonable source, but is preferably available from a nearby gas production field rather than imported from a significant distance. Specifically, if the stream 108 a is hydrogen, it may be provided from an auto-thermal reforming (ATR) process performed on a gas production stream from a nearby production gas field (not shown).

The low heating value fuel gas stream 108 b may be comprised of less than about 80 vol % methane, less than about 60 vol % methane, less than about 40 vol % methane, or even less than about 20 vol % methane. The low heating value stream 108 b may also include small amounts of heavier hydrocarbons such as ethane and propane. In most cases, the majority of the remainder of the stream 108 b will be inert gases such as carbon dioxide, but in some cases, there will be small amounts of nitrogen, hydrogen sulfide, helium, and other gases. Preferably, all non-hydrocarbons and all inert gases other than carbon dioxide will be separated out of the stream 108 b prior to mixing and combustion.

In one exemplary embodiment, the flow and composition of the two hydrocarbon containing streams 108 a and 108 b are used to calculate the oxygen requirement to operate the combustor 110 and provide the set point for the oxygen supply flow controller 118. The calculation will provide the amount of oxygen needed for a stoichiometric combustion in the combustor 110. The flows and compositions of the streams may change over the life of the system 150, depending on the source of the streams 108 a and 108 b. For example, the low heating value fuel gas 108 b may originate from an EOR well having a high methane content in early production (e.g. above about 80 vol %). In such a case, there may be little or no flow through the high quality fuel gas stream 108 a. However, when breakthrough occurs, the flow from the low heating value fuel gas stream 108 b may comprise very low methane content (e.g. less than about 20 vol %). In that case, the flow from the high quality fuel gas stream 108 a is increased to add hydrocarbons to the combustion fuel stream 108.

FIG. 1D is an illustration of a fourth exemplary embodiment of the system of FIG. 1A, which may or may not include the features disclosed in the illustration of the embodiment of FIGS. 1B and 1C. As such, FIG. 1D may be best understood with reference to FIGS. 1A-1C. The system 160 further includes a high concentration CO₂ makeup stream 108 c with a flow and composition and a flow controller 162 operatively attached thereto.

The CO₂ makeup supply stream 108 c may be combined with streams 108 a and 108 b to generate a combustion fuel gas stream 108 having a substantially constant composition over the life of the system 160. The approach is similar to the system 150, but the physical characteristics of the combustor could be designed specifically for the composition of 108 and still burn fuels that have variable composition 108 b. The CO₂ stream 108 c may be split from the expanded products stream 112 b or originate from another source.

FIG. 1E is an illustration of a fourth exemplary embodiment of the system of FIGS. 1A-1D. As such, FIG. 1E may be best understood with reference to FIGS. 1A-1D. The system 170 includes a combustion fuel stream 108 comprising substantially hydrocarbons and carbon dioxide (CO₂) and having an initial fuel to CO₂ ratio; an oxygenation stream 106 comprising substantially oxygen and carbon dioxide (CO₂), wherein the combustion fuel stream 108 and the oxygenation stream 106 are combined to form a combustor inlet stream 172 having a combined fuel to oxygen ratio configured to meet an optimal equivalence ratio and a combined initial CO₂ to fuel ratio configured to provide an optimal combustion temperature; a secondary diluent stream 102 b; and a combustor 110 configured to combust the combustor inlet stream 172 to produce a hot products stream 174 comprising substantially water and carbon dioxide, wherein the hot products stream 174 is mixed with the secondary diluent stream 102 b to form a combustion products stream 112 a having a temperature and a final CO₂ to fuel ratio.

In some exemplary embodiments, the hydrocarbons in the combustion fuel stream 108 are comprised substantially of methane and the fuel to oxygen molar ratio is from about 1.9:1 mol fuel to mol oxygen to about 2.1:1 mol fuel to mol oxygen or from about 1.95:1 mol fuel to mol oxygen to about 2.05:1 mol fuel to mol oxygen. These molar ratios are approximately equivalent to stoichiometric ratios of 0.9:1 to about 1.1:1. In additional exemplary embodiments, the hydrocarbons in the combustion fuel stream 108 are comprised substantially of methane and the final CO₂ to fuel ratio is from about 10:1 mol CO₂ to mol fuel to about 30:1 mol CO₂ to mol fuel or from about 15:1 mol CO₂ to mol fuel to about 25:1 mol CO₂ to mol fuel or from about 20:1 mol CO₂ to mol fuel to about 23:1 mol CO₂ to mol fuel.

In at least one exemplary embodiment, the system 170 further includes a high quality fuel gas stream 108 a with a flow and a composition; a low heating value fuel gas stream 108 b with a flow and composition; and a high concentration CO₂ makeup stream 108 c configured to combine with the high quality fuel gas stream 108 a and the low heating value fuel gas stream 108 b to form the combustion fuel stream 108 and maintain a constant initial fuel to CO₂ ratio of the combustion fuel stream 108. Additional embodiments may include an oxygen supply stream 104 with a flow and a composition; and a high concentration CO₂ mixing stream 102 a with a flow and a composition configured to combine with the oxygen supply stream 104 to form the oxygenation stream 106.

In yet another exemplary embodiment, the system 170 additionally includes at least one temperature sensor 114 configured to measure the temperature of the combustion products stream 112 a and/or expanded products stream 112 b (and optionally the hot products stream 174) wherein the temperature of the streams 112 a or 112 b are used to calculate the flow rate of at least one of the high concentration CO₂ mixing stream 102 a, the high concentration CO₂ makeup stream 108 c, and the secondary diluent stream 102 b, to regulate the temperature of combustion; at least one oxygen analyzer 126 configured to measure the amount of oxygen in the composition of the products streams 112 a and/or 112 b, wherein the amount of oxygen in the products streams 112 a-112 b is used to optimize the flow rate of the oxygen supply stream 104 to achieve substantially stoichiometric combustion; and at least one hydrocarbon analyzer 152 configured to measure the amount of hydrocarbons in the composition of the products streams 112 a-112 b, wherein the amount of hydrocarbons in the composition of the products streams 112 a-112 b is used to optimize the flow rate of the oxygen supply stream 104 to achieve substantially stoichiometric combustion. The system 170 may also include a gas turbine 111 having a load and a load controller 111′ configured to measure the load, wherein the load measurement is used to maintain the combined fuel to oxygen ratio as the load changes.

FIG. 2 illustrates a schematic of an exemplary combustor as it might be configured for use in the alternative exemplary systems of FIGS. 1A-1D. As such, FIG. 2 may be best understood with reference to FIGS. 1A-1D. The combustor system 200 comprises a first mix zone 202, a second mix zone 204, an initial high temperature primary combustion zone 205, a burnout zone 206, and optional fuel injection nozzles 208 a and 208 b. The hot products stream (or “primary combustion products stream”) 212 is produced from primary combustion zone 205. Note, that in some exemplary embodiments, a high pressure combustion process (e.g. greater than about 10 atmospheres) may be utilized.

The first mix zone 202 is configured to mix the primary diluent stream 102 a with the oxygen supply stream 104 to form the oxygenation stream 106. The second mix zone 204 is configured to mix the oxygenation stream 106 and the combustion fuel stream 108. In one embodiment the streams 106 and 108 may be pre-mixed in the second mix zone 204 and then directly flow into the primary combustion zone 205. In another embodiment, which is referred to as non-premixed, the second mixing zone 204 and the primary combustion zone 205 overlap and occur concurrently, as in a diffusion burner type of arrangement. The primary combustion zone 205 includes a burner, a flame, and the combustion reaction itself, which produces the hot products stream 212. The burnout zone 206 delivers the secondary diluent stream 102 b to the combustor 110 to regulate the temperature of the combustor 110 and the combustion products stream 112 a.

In some exemplary embodiments of the combustor 110, the burnout zone 206 comprises one of a passive dilution zone having a series of holes configured to cool and quench the liner of the combustor 110; an active dilution zone having at least one quench port configured to actively deliver at least a portion of the secondary diluent stream 102 b to the combustor 110 to mix with the primary combustion products stream 212; a series of staged quench ports to actively control a temperature pattern through the burnout zone 206; and any combination thereof. In addition, the burnout zone 206 may include a pressure transducer or other sensor 210 to monitor and measure pressure oscillations in the combustor 110, which are a sign of flame blowout. An oxygen analyzer (not shown) may also be included in the combustor 110 to provide another input to the oxygen feedback loop.

In one exemplary embodiment of the non-premixed arrangement, the combustion fuel streams 108 a and 108 b may be introduced in separate nozzles 208 a and 208 b that are configured based on the volume flow of the respective stream, or mixed and injected as a mixed combination of the combustion fuel stream 108 through a single injector.

The combination of the oxygenation stream 106 and the combustion fuel stream 108 in the combustor 110 is configured to maintain a minimum adiabatic flame temperature and flame stability to combust all or nearly all of the oxygen in the oxygenation stream 106 (e.g. a stoichiometric reaction is preferred). In terms of heating value, the oxygenation stream 106 has no heating value, the high quality fuel gas stream 108 a may have a relatively high value (e.g. from at least 500 British thermal units per standard cubic foot (BTU/scf) to about 950 BTU/scf) and the low heating value fuel gas stream 108 b has a relatively low heating value (e.g. from about 150 BTU/scf to about 450 BTU/scf).

The combustor 110 may be a standard external combustor or may be a customized or modified combustor. Examples of applicable combustor types include an oxyClaus burner, a partial oxidation (PDX) burner, auto-thermal reforming (ATR) burner, and gas turbine diffusion combustors. Note that each burner type may require some modification to work with a substantially CO₂ stream.

In one exemplary embodiment, the second mix zone 204 and nozzles 208 a and 208 b may be configured to mix the combustion fuel stream 108 and the oxygenation stream 106 in a highly turbulent manner to ensure a homogeneous mixture is achieved. During operation, the primary combustion zone 205 produces temperatures up to about 2,200° C. With the addition of the quench gas 102 b, the combustion products stream 112 a is expected to be up to about 1,400° C. as it enters the burnout zone 206. Additional quench gas 102 b may be introduced via the outer wall of the burnout zone 206 generating a sort of “gas envelope” to keep the wall of the combustor 110 notably cooler than the flame 205. In one exemplary embodiment, the cooling stream 102 b may be stripped of hydrocarbons to minimize soot formation, if necessary. In another exemplary embodiment, the combustion takes place at higher than atmospheric pressure, such as above about 10 atmospheres. The reaction generates water (vapor) and carbon dioxide as shown by the equations below (the carbon dioxide entering the chamber generally remains unreacted): CH₄+2O₂=2H₂O+CO₂

FIG. 3 is an exemplary flow chart of a method of operating the system of FIGS. 1A-1D. As such, FIG. 3 may be best understood with reference to FIGS. 1A-1D. The method 300 begins at block 302, then includes providing 304 a high concentration CO₂ stream 102, an oxygen supply stream 104, and a combustion fuel stream 108, wherein each stream has a flow rate and a composition; splitting 306 the high concentration CO₂ stream 102 into a primary diluent stream 102 a and a secondary diluent stream 102 b, each having a flow rate; adjusting 308 the primary diluent stream flow rate independently of the overall flow rate of the high concentration CO₂ stream; combining 310 the primary diluent stream 102 a and the oxygen supply stream 104 to form an oxygenation stream (e.g. “synthetic air”) 106; combusting 312 the oxygenation stream 106 and the combustion fuel stream 108 in a primary combustion zone 110 a of the combustor 110 to form a combustion products stream 112 a with a temperature and a composition; sensing 314 the oxygen content of the combustion products stream 112 a; and adjusting 316 the flow rate of the oxygen supply stream 104 until the combustion products stream 112 a is substantially stoichiometric using the sensed oxygen content. In one embodiment, the method 300 includes sensing 314 the temperature of the combustion products stream 112 a and adjusting 316 the flow rate of the primary diluent stream 102 a to regulate the temperature of combustion using the sensed temperature. In one embodiment, the method includes directing 318 a portion of the combustion products stream 112 a to an enhanced oil recovery (EOR) process.

EXAMPLES

Some exemplary gas stream compositions are provided in the tables below as examples of gas streams at different stages of production in a single gas production field, or different gas production fields. Table 1 provides specific stream compositions and flow rates for a production well at or near the beginning of production.

TABLE 1 Start-up Stream Stream Stream Stream Stream Component 104 108b 108a 102a 112 O₂ 95.59% 0 0 0 0.44% CO₂ 0 0 0 76.23% 61.83%  CH₄ 0 0   100% 0 0.00% CO 0 0 0 0 0.85% OH 0 0 0 0 0.12% H₂ 0 0 0 0 0.09% H₂O 0 0 0 16.99% 30.42%  Ar  4.26% 0 0  6.78% 6.34% Misc  0.15% 0 0 0 0.00% Total 100.00%  0.00% 100.00% 100.00%  100.09%  Pressure psig 300 300 300 300 250 Temp (° F.) 755 500 160 540 1701.7 LB Moles 13474.1 0 6464.1 143859.5 163798 Flow (lb/hr) 436010 0 103425 6282874 6822309 Table 2 provides specific stream compositions and flow rates for a production well after CO₂ breakthrough.

TABLE 2 Post Breakthrough Stream Stream Stream Stream Stream Component 104 108b 108a 102a 112 O₂ 95.59% 0 0 0 0.014% CO₂ 0 88.16%  0 0 64.15% CH₄ 0 5.21%   100% 0  0.00% C₂ 0 2.76% 0 0  0.00% C₃ 0 1.25% 0 0  0.00% CO 0   0% 0 0 0.028% OH 0   0% 0 0 0.004% H₂ 0   0% 0 0 0.236% H₂O 0   0% 0 0 31.02% N₂ 0   1% 0 0  0.84% Ar  4.26% 0 0 0  0.40% Misc  0.15% 1.77% 0 0  3.3% Total 100.00%  100.00%  100.00% 0.00% 100.09%  Pressure psig 300 300 300 300 250 Temp (° F.) 755 500 160 540 1701.7 LB Moles 13474.1 136739.7 171.8 0 150386 Flow (lb/hr) 412653 5639146 2748 0 6054547

While the present invention may be susceptible to various modifications and alternative forms, the exemplary embodiments discussed above have been shown only by way of example. However, it should again be understood that the invention is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present invention includes all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims. 

What is claimed is:
 1. A system, comprising: a combustor that includes a combustion fuel port, an oxygen supply port, a diluent supply port for providing a primary diluent, a combustion products output port, and a primary combustion zone and a burnout zone, wherein the combustor is configured to mix and combust an oxygenation stream and a combustion fuel stream within the primary combustion zone at a flame temperature and a primary residence time sufficient to produce a hot products stream, and configured to dilute the hot products stream with a secondary diluent stream within the burnout zone to form a combustion products stream having a lower temperature than the hot products stream, wherein the primary diluent and the secondary diluent have high concentrations of carbon dioxide and are split from a recycle stream; an oxygen analyzer that measures an amount of oxygen in the combustion products stream after exiting the combustor through the combustion products output port; and a control system comprising: a first controller configured to receive a signal from the oxygen analyzer to thereby regulate the flow rate of an oxygen supply stream, and a second controller which controls a flow rate of the combustion fuel stream entering the combustor through the combustion fuel port, wherein the first controller and second controller adjust a proportioning factor based on the amount of oxygen in the combustion products stream determined by the signal from the oxygen analyzer to maintain an oxygen-to-fuel ratio in the combustor that creates near stoichiometric combustion; a third controller configured to regulate the oxygen to CO2 ratio of the oxygenation stream by controlling the primary diluent stream flow rate to produce a primary combustion zone flame temperature configured to provide: an adequate margin between a combustor average velocity, a blow-off limit of the combustor, and the primary residence time within the combustor sufficient to produce the combustion products stream at about equilibrium conditions; at least one temperature sensor operatively connected to an at least one CO2 flow regulation device, wherein the at least one temperature sensor is configured to measure the temperature of the combustion products stream and send a signal to the at least one CO2 flow regulation device, whereby the control system adjusts the secondary diluent stream flow rate based on the temperature of the combustion products stream to produce a desired temperature of the combustion products stream; and an expansion device that receives the combustion products and expands the combustion products, whereby the expansion device is operatively connected to at least one load controller configured to control the flow rate of the combustion fuel stream to maintain a desired load condition in the expansion device.
 2. The system of claim 1, wherein the temperature sensor measures the temperature of the combustion products stream after exiting the combustor through the combustion products output port.
 3. The system of claim 1, wherein the control system controls the flow rate of the oxygen supply stream independent of an adjustment to the primary diluent supply stream.
 4. The system of claim 1, wherein the control system adjusts the proportioning factor by adjusting the flow rate of the oxygen supply stream.
 5. The system of claim 1, wherein a substantially stoichiometric combustion results in the combustion products stream including less than about 3.0 volume percent (vol %) oxygen and less than about 3.0 volume percent (vol %) NOx.
 6. The system of claim 1, wherein the control system controls a desired combustor stoichiometry based on a condition of a load communicatively coupled to the combustion products output port changes.
 7. The system of claim 1, wherein the oxygen analyzer is a lambda oxygen sensor.
 8. The system of claim 1, wherein the oxygen analyzer is a wideband zirconia oxygen sensor. 